(1) Field of the invention:
This invention relates to an epitaxial growth substrate comprising a base material composed of a sapphire substrate, a SiC substrate, a GaN substrate or the like and an AlxGayInzN film (x+y+z=1,x greater than 0,y,zxe2x89xa70) on the base material, and a method for producing the same.
(2) Related Art Statement:
An epitaxial growth substrate is employed for an electronic device such as a light emitting diode (LED), a laser diode (LD) or a field effect transistor (FET). In producing such an electronic device, semiconductor films such as AlaGabIncN film (a+b+c=1,a,b,cxe2x89xa70) are formed on the epitaxial growth substrate by epitaxial growth. Attention is paid to an AlaGabIncN film (a+b+c=1,a,b,cxe2x89xa70) because it has a large bandgap to generate and emit a short wavelength light in a light emitting element.
FIG. 1 is a cross sectional view showing a conventional light emitting diode to generate a blue light, which is composed of the above AlaGabIncN films.
For example, a GaN film 2 as a buffer layer is formed on a C-faced sapphire (Al2O3) substrate 1 at a low temperature by CVD, and an n-type AlaGabIncN film 3 is formed on the GaN film 2 by epitaxial growth through CVD. Then, a p-type AlaGabIncN film 4 is epitaxially grown on the n-type AlaGabIncN film 3 by epitaxial growth through CVD, and a low resistive p-type AlaGabIncN film 5 is epitaxially grown on the p-type AlaGabIncN film 4.
Electrodes 6 and 7 are formed on the n-type AlaGabIncN film 3 and the p-type AlaGabIncN film 5, respectively.
If the n-type AlaGabIncN film 3 is directly formed on the sapphire substrate 1 by CVD, it has a large amount of defect, its depleted crystallinity and its rough surface. Therefore, in this case, the light emitting diode having the directly formed n-type AlaGabIncN film can have a relatively small light emission efficiency.
Therefore, as shown in FIG. 1, the GaN film 2, which is formed at a low temperature by CVD, intervenes as the buffer layer between the sapphire substrate 1 and the n-type AlaGabIncN film 3. Since the GaN film 2 is epitaxially grown at a low temperature by CVD, the lattice constant difference of approximately 10% between the sapphire substrate 1 and the n-type AlaGabIncN film 3 is compensated, and the AlaGabIncN film can have its flat surface, which is very important in a heterojunction of the light emitting diode.
Moreover, it is proposed that an AlN film, which is epitaxially grown at a low temperature by CVD, is employed as the buffer layer instead of the GaN film 2.
However, if the AlaGabIncN film incorporating Al as an indispensable component is epitaxially grown on the GaN film or the AlN film formed at a low temperature by CVD, it has a large amount of dislocation, for example a dislocation density of 1010/cm2.
Since the large amount of dislocation forms light absorption centers, a semiconductor device having the AlaGabIncN film with the large amount of dislocation is degraded. It is a large problem in an optical device such as a laser diode requiring a large light emitting efficiency. Moreover, the large amount of dislocation deteriorates the condition of the pn junction of the optical device or the like. Consequently, it is very important to reduce the dislocations.
In view of the reduction of the dislocations, a selective lateral epitaxial growth method is disclosed in xe2x80x9cJ. Appl. Phys, Vol. 68, No. 7, p.774-779xe2x80x9d. In the document, the selective lateral epitaxial growth method is employed in epitaxially growing a GaN film on a sapphire substrate. Therefore, the inventors of the present invention make an attempt to epitaxially grow the AlaGabIncN film by taking advantage of the above selective lateral epitaxial growth method.
In this case, as shown in FIG. 2a, a GaN film 12 is epitaxially grown in a thickness of 1-2 xcexcm on one main surface of a sapphire substrate 11 by CVD, and thereafter, a strip-shaped mask 13 is formed of a SiN material on the GaN film 12. Then, a AlxGayInzN films (x+y+z=1,x,y,zxe2x89xa70) 14 are epitaxially grown on the GaN film 12.
As shown in FIG. 2b, the Alx,GayInzN films 14 are epitaxially grown on the GaN film 12 alongside the mask 13, and then, are laterally grown on the mask 13 as they reach the upper surfaces of the mask 13. Lastly, as shown in FIG. 2c, the laterally grown AlxGayInzN films are coalesced on the mask 13.
In this case, the dislocations at the interfaces between the GaN film 12 and the AlxGayInzN films 14 are transmitted longitudinally, not laterally on the mask 13. Therefore, the AlxGayInzN films 14 have respective small amounts of dislocation, for example dislocation density of about 107/cm2, within areas W on the mask 13.
Therefore, if a light emitting element as shown in FIG. 1 is formed on the low dislocation density area W, it can exhibit good performances.
In the above conventional method, however, the sapphire substrate 11 is set into a CVD chamber, and then, the GaN film 12 is formed on the sapphire substrate 11. Thereafter, the sapphire substrate 11 is taken out of the CVD chamber, and the strip-shaped mask 13 is formed on the GaN film 12 by photolithography. Then, the sapphire substrate 11 is set into the CVD chamber again, and the AlxGayInzN films 14 are formed.
That is, since the conventional method requires the above complicated process in forming the AlxGayInzN films 14, it can not provide an epitaxial growth substrate efficiently.
Moreover, in forming step of the mask 13 after forming the GaN film 12 as a buffer layer, impurities are incorporated into the GaN film, resulting in the degradation of the properties of the GaN film.
The above phenomenon can be observed in the case of forming the GaN film on a SiC substrate or a GaN substrate instead of the sapphire substrate. Consequently, the above conventional method can not provide an epitaxial growth substrate having the above AlxGayInzN film, particularly the AlxGayInzN film (x+y+z=1,x greater than 0,y,zxe2x89xa70) incorporating Al as an indispensable component with low dislocation density and good properties, efficiently.
It is an object of the present invention to provide an epitaxial growth substrate comprising a base material composed of a sapphire substrate, a SiC substrate, a GaN substrate or the like and an AlxGayInzN (x+y+z=1,x greater than 0,y,zxe2x89xa70) film with low dislocation density and good properties thereon, on which AlaGabIncN (a+b+c=1,a greater than 0,b,cxe2x89xa70) films having good properties can be formed by epitaxial growth.
It is another object of the present invention to provide a method for producing the above epitaxial growth substrate.
This invention relates to an epitaxial growth substrate comprising a base material having
a strip-shaped concave-convex structure thereon and an epitaxially grown AlxGayInzN (x+y+z=1,x greater than 0,y,zxe2x89xa70) film formed so as to embed the concave-convex structure by a selective lateral epitaxial growth method and having low dislocation density areas on at least one of the concave portions and the convex portions of the concave-convex structure.
In a preferred embodiment of the present invention, the above concave-convex structure may be composed of plural kinds of strip-shaped ditches formed on the surface of the base material.
In the epitaxial growth substrate of the present invention, the cross sectional shape, size, ditch direction and aspect ratio of the above concave-convex structure can be determined appropriately in accordance with the composition and forming conditions of the AlxGayInzN film and a kind of device to be produced.
Moreover, this invention relates to A method for producing an epitaxial growth substrate comprising the steps of:
preparing a base material,
forming a strip-shaped concave-convex structure on a surface of the base material, and
epitaxially growing an AlxGayInzN (x+y+z=1,x greater than 0,y,zxe2x89xa70) film so as to embed the concave-convex structure by a selective lateral epitaxial growth method, whereby the AlxGayInzN film has low dislocation density areas on at least one of the concave portions and the convex portions.
According to the producing method of the present invention, since the base material having the concave-convex structure, for example composed of plural strip-shaped ditches is set into a CVD chamber, and then, the AlxGayInzN film is laterally and epitaxially grown, the above epitaxial growth substrate can be provided efficiently.
Moreover, since the AlxGayInzN film to constitute the epitaxial growth substrate is not influenced by a mask-forming process as in the above conventional method, an epitaxial growth substrate having good properties can be provided.